This invention relates to two new families of high velocity oxygen fuel (HVOF) thermal spray superalloy (MCrAlYxe2x80x94X) alloy powders.
Current MCrAlY compositions have been mostly designed to be used as cast metals for high temperature (850-1100xc2x0 C.) applications on components such as blades and vanes on aircraft and land-based turbines. MCrAlY alloys are commonly used at these elevated temperatures due to their ability to form an oxide scale, either alumina or chromia based, which acts as a protective barrier against high temperature oxidation and corrosion environments. The adherence and stabilization of this scale is directly related to the useful life of the alloy. Scales that have poor integrity and xe2x80x9cflake-offxe2x80x9d cause the continual outward diffusion of key elements in order to replace the exposed area with new oxide scale. This process eventually depletes the alloy of its critical elements thereby reducing its high temperature effectiveness.
Accordingly, the main object of the invention is to provide HVOF powders that offer improved high temperature oxidation, corrosion and/or creep resistance.
The foregoing and other objects of the present invention are achieved, at least in part, by utilizing gas atomized MCrAlYxe2x80x94X alloy compositions consisting of a metal, M, of nickel, cobalt and/or iron, chromium with levels between 20-35 wt %, aluminum with levels between 5-15 wt %, yttrium with levels greater than 1 wt % and up to about 6 wt % and a reactive element, X, which can be hafnium, rhenium, lanthanum and/or tantalum with levels between 2-10 wt %.
The compositions are unique in that they are designed to be used as overlay coatings and deposited via the HVOF process. The chemistries of these alloys have been especially tailored to compensate for the relatively low temperature, oxygen-rich environment of the HVOF combustion flame.
The addition of a reactive element to an MCrAlY composition has been found to enhance the adhesion of the protective oxide scale, increase the plasticity of the scale, grade the mechanical differences between the ceramic oxide scale and the metallic base alloy, and modify the scale growth mechanisms by controlling the diffusion process.
The role of yttrium in the composition is to help adhere the protective oxide scale to the coating structure below. However, it has been found that traditional yttrium levels (1 wt % or less) are insufficient when deposited with the HVOF process. This insufficiency is due to the oxide formation of the yttrium forming various phases of yttria, which are less beneficial to the oxide adhesion process. In order to compensate for this occurrence, the present invention includes a new family of MCrAlY compositions that contain a higher level of yttrium (greater than 1 wt % and up to about 6 wt %, preferably 1.5 wt % to 5.5 wt %, and most preferably about 4 wt %) in the powder. This increased yttrium level in conjunction with the addition of a reactive element, produce an overlay that is better able to withstand high temperature oxidation and corrosion environments by improving the formation and adherence of the protective oxide scale.
For optimal performance, MCrAlY coatings must be sprayed with a xe2x80x9ccoolxe2x80x9d HVOF flame in order to minimize the formation of internal oxides. This limited heat content of the flame also minimizes the effectiveness of depositing high melting point materials like ceramics. It has been found that by surrounding the ceramic particles with lower melting point metallic alloys, a higher degree of ceramic can be entrapped within the coating. These discrete ceramic particulates within the coating help to restrict grain growth within the alloy at high temperatures. This restriction in turn improves the material""s creep resistance.
Thus, in a first family of powders in accordance with the present invention, a MCrAlYxe2x80x94X composition is specifically designed for the HVOF deposition process. The yttrium content has been increased to greater than 1 wt % and up to about 6 wt % to compensate for the oxidation that occurs during deposition. The reactive element component, X, is incorporated to improve the formation and adhesion of the protective oxide scale. This improved adhesion leads to increased oxidation and corrosion protection offered by the overlay.
The present invention also includes a second family of HVOF MCrAlYxe2x80x94X materials, which is a blend of (1) MCrAlYxe2x80x94X, and (2) MCrAlY and ceramic, that have been designed to offer superior creep resistance over traditional alloy compositions. This class of alloys contains an agglomerated (spray dried) secondary component consisting of MCrAlY+ceramic (i.e., alumina, yttria, garnet, etc.). Thus, in addition to the advantages of the improved oxide scale properties from the MCrAlYxe2x80x94X component, this class takes advantage of the improved creep resistance provided from the addition of an agglomerated ceramic component. The agglomerated secondary component is present in the blend in levels of 30-50% by weight of the total composition. The overall amount of ceramic in the bulk powder ranges from 3-15% by weight, preferably 4.5-7.5 wt %. In a preferred embodiment, an agglomerated powder with 15 wt % Al2O3 and 85% CoNiCrAlY is blended with MCrAlYxe2x80x94X powders. Again, this class of materials is unique in that the addition of a secondary component, which is an agglomeration of a ceramic and a metal, takes advantage of the HVOF flame characteristics.